Certain items of aircraft equipment, for example electronic actuators, may in use require cooling. Cooling of such equipment may be effected by means of transporting cooling fluid to the equipment by means of a duct. In the case where the item of equipment is bulky, as may be the case when the equipment is an electrically powered hydraulic actuator for controlling ailerons or spoilers on a wing, the free space available for a duct may be limited. Some of the factors, insofar as incorporating such a cooling duct is concerned, that need to be taken into account when designing an aircraft structure will now be described. Preferably, such ducts are placed in a location such that maintenance and/or removal of other components in the aircraft can be performed without needing to perform a separate step of moving or removing the duct or a part thereof. Preferably, such ducts are predominantly straight, will avoid sudden changes of direction, or contain at most one or two turns, otherwise the effective flow of cooling fluid can be impaired. Preferably, the duct is rigid. Preferably, if the duct is fed with air from the exterior of the aircraft via an air inlet, such an air inlet should be outside of any designated fire zones. Preferably, when designing the shape and dimensions of an aircraft structure, the need to supply a cooling fluid duct should not have any significant effect on the design. It would for example be extremely disadvantageous for the volume of an aircraft structure to be increased during the design process simply to accommodate a cooling fluid duct. For example, it is preferred for any ducting to be contained within the volume of the aircraft structure as designed without taking into account the need to accommodate such ducting. Preferably, the flow of cooling fluid into and along the duct is effected without the need for additional pumps, fans or the like. Preferably, the mass and length of the duct should be kept as low as is reasonably practical. Preferably, in the case where the duct is to be located in an area of a wing, which may be congested with existing systems, the cooling duct should be positioned so as there is no disruption or re-routing of such systems. There are therefore many factors that need to be taken into account when designing an aircraft structure that includes a cooling fluid duct. Conventional ducts may in certain circumstances be difficult to incorporate into the design of an aircraft structure whilst taking into account the design factors mentioned above. It will of course be appreciated that in many cases, including the present invention, the above factors to be taken into account are optional and one or more factors may be given less weight than others, or even be ignored, in relation to a particular design.